1. Field of the Invention
The present invention relates to compensation for recording medium unevenness during printing operations. More specifically, the present invention relates to control over timing of ink droplet ejection to compensate for recording medium unevenness.
2. Description of the Related Art
Recording medium (paper) unevenness is a known phenomena in ink-jet printing operations. The recording medium unevenness (sometimes called xe2x80x9ccocklingxe2x80x9d) is caused by excessive wetting of the paper by the liquid ink. The cockling introduces an unknown waveform shape into the paper that causes problems during printing operations, such as interference with a recording head during scanning. That is, high spots in the waveform shape of the paper interfere or rub against the recording head as it scans across the paper. The interference can cause problems such as clogging of the ink nozzles on the recording head and smearing of the ink.
To minimize interference problems caused by cockling, it has been proposed to apply pressure to the paper ahead of the recording head as it scans across the paper. One way this has been done is to provide a smaller roller on the printer carriage ahead of the print head such that, as the roller scans across the paper, the roller flattens the uneven paper ahead of the print head. However, the roller only slightly reduces the amount of cockling in the paper and after the roller flattens the paper, the paper tends to return to its uneven condition. Therefore, although the roller somewhat reduces the possibility of interference with the recording head, other problems associated with paper cockling still exist.
Another problem associated with paper cockling is image roughness that is caused by an uneven spacing of the ink droplets as they contact the paper. The ink droplet spacing is dependent upon several factors, including the carriage speed, the ink ejection speed and the distance between the print head and the paper. As seen in FIGS. 13A and 13B, ink droplets are ejected by the recording head at a constant frequency (f) along the scan direction. If the paper is flat or at least very close to being flat as seen in FIG. 13A, the ink droplets contact the paper at approximately the same spacing (d). However, when cockling occurs in the paper and the paper takes on a waveform shape as seen in FIG. 13B, the ink droplets do not contact the paper with a constant spacing, but rather they contact the paper with a different and varying spacing. That is, although the ink droplets are ejected by the recording head at a constant frequency f, the waveform shape of the paper causes some of the ink droplets to contact the paper in a more narrow pattern (d1) than they were ejected at, and some of the ink droplets to contact the paper in a wider pattern (d2) than they were ejected at. Thus, the waveform shape effects the contact frequency because of the varying distance between the print head and the paper. As a result, even though the ink droplets were ejected at a constant frequency, the spacing between the ink droplets contacting the paper is not the same as the spacing frequency that they were ejected at and image roughness occurs.
This problem is made worse in bi-directional printing modes. In bi-directional printing, a line of ink droplets is printed in a forward scan of the recording head, the paper is advanced one line and then another line of ink droplets is printed in a reverse scan of the recording head. Therefore, in bi-directional scanning, the ink droplet frequency contacting the recording medium varies from line to line, which makes the image roughness even worse than unidirectional scanning.
The inventors herein have considered the foregoing problem and have considered a method to compensate for the varying contact frequency of the ink droplets by varying the frequency of ejecting the ink on a region by region basis. In somewhat more detail, FIG. 19 depicts a method considered by the inventors herein for compensating for the contact frequency discrepancies wherein the waveform shape of the paper is divided into a predetermined number of regions and control over the firing frequency is performed by an ASIC. Within each region (intra-region), the ink ejection frequency is set to the same value for the entire region. However, the ink ejection frequency between regions (inter-region) is varied from region to region. It has been found that this approach works well in compensating for the paper cockling, but the inventors herein have also determined that a different approach may be utilized to provide the compensation. As such, the present invention is different from the foregoing approach considered by the inventors herein.
The present invention addresses the foregoing by inducing a predetermined unevenness pattern into the recording medium, determining an adjusted ink ejection frequency based on the induced unevenness pattern and adjusting the frequency of ink droplet ejection at each position of a print head scan across the recording medium based on the adjusted frequency. As a result, the ink ejection frequency can be adjusted by a CPU at each print head scanning position to compensate for the known unevenness pattern. Therefore, ink droplets contact the recording medium in a more even spacing along a scan direction and image density roughness that would otherwise occur is reduced.
Accordingly, in one aspect the invention may be control of an ink ejection frequency to compensate for recording medium unevenness in printing by inducing a predetermined unevenness pattern into the recording medium, determining an adjusted ink ejection frequency for each of a plurality of print head scan positions for a scan of the print head across the recording medium, the adjusted ink ejection frequency being determined at least in part based on the induced unevenness pattern, adjusting a base ink ejection frequency for each scan position of the print head based on the determined adjusted ejection frequency, and controlling ink ejection by the print head based on the adjusted ink ejection frequency.
The determined adjusted ink ejection frequency may be stored in a storage medium in the form of a look-up table with the adjusted ink ejection frequency being obtained from the look-up table. In addition, a plurality of look-up tables corresponding to a plurality of recording medium types and printing modes may be stored in the storage medium, with the adjusted ink ejection frequency for each print head scan position being obtained from the respective look-up table based on a recording medium type and a printing mode selected by a user. The control of the ink ejection frequency is preferably performed by a CPU in the printing device.
The invention may be implemented with multiple print heads and in bi-directional printing. The multiple print heads may be controlled individually based on the color of ink that the print head ejects, as well as based on whether the print head is scanning in a forward or reverse direction.
Each print head can be controlled with the same control signal, especially if the print heads are spaced relative to one another a distance corresponding to the spacing between the cockling ribs. Spacing the print heads relative to one another a distance corresponding to the distance between the cockling ribs allows both color and black print data can be compensated for accordingly with the same control signal. However, if the print heads are not spaced relative to one another a distance corresponding the distance between the cockling ribs, then if color and black data are to be printed, the color print head may be controlled, and if only black data is to be printed, the black print head can be controlled. Additionally, bi-directional compensation can be provided for, thereby resulting in less density unevenness of mixed color images as well as bi-directional printed images.
The invention may further detect a distance between the print head and the recording medium as the print head scans across the recording medium and utilize the detected distance in determining an adjusted ink ejection frequency.